Solid state digital to analog converter

ABSTRACT

A digital-to-analog converter comprising an IC switch module providing four switch transistors and associated switch-control buffering circuitry. The emitter areas of the switch transistors are binarily weighted to provide equal current densities. The IC substrate also is formed with a fifth transistor to serve as a reference transistor for adjusting the supply voltage as necessary to maintain constant current through the switch transistors. To construct a digital-to-analog converter having a high bit resolution, a number of such &#39;&#39;&#39;&#39;quad&#39;&#39;&#39;&#39; switch modules may be combined, for example in a printed circuit card assembly including a thin-film resistor module providing binarily-weighted resistors on a glass substrate to set the current levels through the switch transistors.

United States Patent [1 1 [11] 3,747,088 Pastoriza July 17, 1973 SOLID STATE DIGITAL-TO-ANALOG' 3,588,530 6/1971 Langan 340/347 DA CONVERTER P E I D l w C k rzmary xammerary I 00 [75] lnvemor' a Putin-ll" cambndge Assistant Examiner-Joseph M. Thesz, .lr.

Attorney-Bryan, Pal-melee, Johnson & Bollinger [73] Assignee: Analog Devices Inc., Cambridge, I i

v Mass. [57] ABSTRACT [22] Fied; 30, 1970' A digital-to-analog converter comprising an IC switch module providing four switch transistors and associated PP N04 102,854 switch-control buffering circuitry. The emitter areas of a the switch transistors are binarily weighted to provide 52 US. Cl. 340/347 DA equal The substrate is 51 Int. Cl. ..;...-H03k 13/02 with fifth as 8 581 Field of Search 340/347 DA, 347 cc; f adjusting supply 8= as Y main- 4 V 307/279 303 tam constant current through the switch transistors. To t construct a digital-to-analog converter having a high bit [56] Rehnnc cued resolution, a number of such quad" switch: modules 5 may be combined, for example in a printed circuit card UNITED T S PATENTS assembly including a thin-film resistor module provid- 3'475'749 10/1969 f 340/347 DA ing binarily-weighted resistors on a glass substrate to g set the current levels through the switch transistors. 3:262:049 7/1966 Watson et al. 340/347 DA 8 Claims; 8 Drawing Figures PAIENIEUJUUTW 3.147. 088

74 (GLASS SUBSTRATE) INVENTOR. James fPas tom/zap ATTORNEY PAYENTEBJUL l mu SHEUMUF I 55:0 3:523 57 SI :29. 5E

REP

' This invention relates to digital-to-analog converters. More particularly, this invention relates to such converters based on solid-state electronics.

Digital-to-analog (D-A) converters have been required for a wide variety of purposes, such as transforming the digital outputs of a high-speed computer to corresponding analog voltages. D-A converters also are used in analog-to-digital converters. A wide variety of D-A converterdesigns have been proposed, and a number have been sold commercially. A particularly successful design is disclosed in co-pending application Ser. No. 809,700 filed by the present inventor on Mar. 24, 1969. Reference to that earlier application is hereby made for details of certain design features which are also incorporated in the embodiment of the present invention to be described below, and the priority date of that earlier application is herein asserted for such common subject matter.

With the development of integrated-circuit (IC) technology, efforts have been made to produce D-A converters in IC form, seeking the benefits of improved reliability, small size, low power consumption, and low production costs. However, prior IC converter designs have not suitably adapted IC technology to the special requirements of D-A converters, and have not satisfactorily exploited the real potential of such technology.

It is therefore-one goal of the present invention to provide D-A converter designs which achieve superior tors so as to effect nearly perfect compensation for the variables which can cause errors in the analog output signal.

Accordingly, it is an object of this invention toprovide a superior D-A converter design based on lC technology. A further object of this invention is to provide a D-A converter having markedly improved performance characteristics. Still another object of this invention is to provide a D-A converter which can be manufactured economically, and which can flexibly be applied to meet diverse requirements. Other objects, aspects, and advantages of the invention will in part be pointed out in, and in part apparent from, the following description considered together with the accompanying drawings, in which:

FIG. 1 shows schematically the circuit arrangement of a quad-switch module and its associated resistance network;

FIG. 2 is a schematic plan view of the portion of the IC chip (substrate) carrying the switching transistors of the quad-switch module of FIG. 1; i FIG. 3 is a cross-section taken along line 33 of FIG. 2, showing the layers defining the different transistor segments;

FIG. 4 shows schematically how three identical quadswitch modules are combined with a resistor module to form a 12-bit D-A converter;

FIG. 5 is a schematic diagram showing a compensation circuit for stabilizing the converter output with changes in ambient temperature and/or other variables;

characteristics from a unique adaptation of integratedcircuit concepts and processing.

In a preferred embodiment of the invention to be described hereinbelow in detail, there is provided a D-A converter comprising a number of separate interconnected modules. The basic module of this converter is an IC switch unit having a single monolithic substrate in which has been diffused a number of switching transistors, together with associated control and logic circuitry for selectively activating the switches in accordance with a digital input signal. One or more such switch modules are assmelbed on a printed circuit board together with a resistor module having a set of precision metering resistors which fix the level of current through the switching transistors in a binary weighting pattern.

The preferred switch module arrangement provides four switching transistors, and thus has been called a quad-switch. D-A converters having resolutions of 4, 8, 12 or 16 bitscan readily be provided, in a flexible manner, simply by using one, two, three or four identical quad-switch modules. Current-dividers of 16:1

attenuation ratio (or 10:1 for binary coded decimal) are used to reduce the current levels from the second, third or fourth switch modules to obtain the correct current contribution for each bit of the digital input.

The conductive areas of the switching transistors of each'quad-switch module are binarily weighted in the ratio of 8:4:2:l. Thus each conductive area is directly proportional to the current carried by the associated transistor, so that the current density is the same for all switching transistors. This provides important benefits,

including superior tracking and lower offset differences between switches. Each switch module substrate also is formed with an additional reference transistor which controls the supply voltage for all the switching transis- FIG. 6 is a schematic diagram illustrating a modified form of compensation circuit;

FIG. 7 is a wiring diagram of a 12-bit D-A converter; and

FIG. 8 shows the physical arrangement of the principal components making up the D-A converter illustrated in FIG. 7.

Referring now to FIG. 1, a quad-switch module 10 is schematically illustrated as a single monolithic sub-' strate 12 having diffused therein four. seprate switching transistors generally indicated at 14, 16, 18 and 20. Each transistor is represented symbolically as a number of parallel-connected sub-transistors, with the numbers of sub-transistors differing in accordance with a binary weighting pattern, i.e., in a ratio of 824:2:1.

The switching transistors are arranged as current sources, and the outputs of all of the transisotrs are connected together to a common line 22. The transistors are selectively activated by respective switch control and logic circuits comprising buffer transistors 24, 26, 28, 30 controlled through corresponding diodes 34, 36, 38, 40 in accordance with the respective bits of a digital input signal. A detailed description of such a switch control circuit is set forth in the abovementioned co-pending application Ser. No. 809,700.

The first transistor 14 provides a current output level representing the most-significant bit (MSB) of the digital input signal, the next transistor 16 provides half as much current representing the second-mostsignificant-bit,-and so forth. The current level in each case is determined by a respective current-weighting metering resistor 44, 46, 48, 50 forming part of a separate resistor module 52. The ohmic resistancesof these resistors may, for example, be 10K, 20K, 40K and K, respectively, to produce switch currents of 1 ma., 0.5 ma., 0.25 ma., and 0.125 ma.

Referring also to FIGS. 2 and 3, all of the transistors 14-20 have collector, base, and emitter segments diffused in the substrate 1.2. The conductive areas of these transistors are binarily weighted to correspond with the current flowing through the transistor. Specifically, the total areas of the respective emitters 54, 56, 58, 60 are in direct proportion to the currents to be carried by the corresponding transistors.

In the preferred embodiment, the emitter areas are so proportioned by providing for each transistor a predeterminednumber of equal-area emitters 54A, 54B, 56A, 56B, etc., with the number of emitters for each transistor being in accordance with a binary weighting pattern. Thus, the right-hand transistor 20 includes a single emitter 60 of preselected area, the next transistor 18 includes two emitters 58A, 58B of that preselected area, and so forth. The size of the base segment 64, 66, 68, 70 of each transistor is roughly proportioned to the surrounding emitter area, and a single common collector segment 72 is provided for all transistors of the switch module.

By proportioning the emitter areas of the switching transistors l4-20 to the level of current to be carried, the current density is made uniform throughout the conducting regions of all transistors. This results in essentially equal base-to-emitter voltages (V for all of the switching transistors. Initial offset between switches, as well as offset drift between switches, are minimized by this arrangement, so as to provide improved accuracy. Because all transistors are diffused on a single common substrate, the transistor characteristics (particularly Beta) will be effectively matched for all switches, further minimizing errors in the output due to changes in variables such as temperature.

The resistor module 52 is assembled together with the switch module by interconnecting means, preferably a single printed circuit board. The resistor module comprises a non-conductive substrate 74 (glass or the like) on which is deposited by known techniques (e.g., sputtering) a thin film of a metallic substance such as nichrome, arranged to form a set of resistors having the required ohmic resistance. Resistors of this type have very low temperature coefficients, so that there will be only small changes in current with changes in temperature. By having all of the resistors on a single substrate, and formed of the same material, any changes in resistance will be proportionately the same for all resistors, thereby assuring uniform performance characteristics for the individual switches.

FIG. 4 shows schematically a 12-bit D-A converter composed of three identical quad-switch modules 10A, 10B, 10C assembled with a binary resistor module 52 as described above. The resistor network includes three identical sets 80A, 80B, 80C of four current-weighting resisotrs 44, 46, 48, 50. The resistors of each set are connected to the respective transistors l4, 16, 18, 20 of a corresponding quad-switch module as described above. The resistor module also includes currentdividing networks 82, 84 which attenuate the currents from the second and third quad-switch modules by factors of 16:1 and 256:].

One important advantage of the module construction described above is that the range of required resistance values is relatively low. For example, in the preferred quad-switch module, the resistance range required is only 8:1, a range well suited for commercial processing. In comparison, a conventional straight l2-bit converter would require a resistance range of 2048zl, e.g., a 10K resistor for the M88 and a 20 megohm resistor for the LSB.

It is difficult to make resistors of such widely different values as 10K and 20 megohm from the same resistance alloy. On the other hand, if different materials are used for the high and low value resistors, they will have different temperature coefficients, and hence will introduce tracking errors with changes in operating temperature. Tracking errors lead to degraded linearity and loss of monotonicity. Loss of linearity results in unequal incremental steps. Loss of monotonicity means that, at some points, the output current will decrease in response to an increased digital number.

These problems and difficulties are avoided or substantially minimized by the module construction described hereinabove, wherein the range of resistance values required for each switch module is relatively small, and the resistors are formed by thin film deposition on a single substrate. Moreover, the modular concept permits the current through the LSB transistor switch to be large relative to the transistor leakage. For example, in the preferred four-transistor quadswitch package, the smallest switch current is 0.125 ma., well above the transistor leakage level. In comparison, the LSB current in a straight l2-bit converter may be as low as 500 nanoamps, which begins to approach the transistor leakage value and hence may well be subject to switching error.

Another advantage of the modular concept described hereinabove is that it simplifies production testing. A quad-switch, for example, need be tested only for 16 different codes. Thus a total of only 48 code tests need be made for the three quads used to make up a 12-bit converter. In comparison, a conventional straight converter may require testing by 1,000 or so codes, leading to higher manufacturing cost.

The modular concept provides another benefit in manufacturing, because it divides the IC processing into smaller but identical units. In the IC process, the yield (i.e., the proportion of acceptable units) is generally proportional to the size of a unit, whereas the cost is generally proportional to the square of the size. By sub-dividing the complete device into a set of identical smaller units, the overall manufacturing operation is made more effective.

The module arrangement described herein lends itself well to standard commercial packaging configurations, such as the DIP pack (14 or 16 pin) or the socalled flat-pack. Thus the converter is well adapted for flexible application to many different types of electronic apparatus requirements.

The provision of exactly four switching transistors in each module is an especially advantageous feature. This number of switches affords an'apparently optimum balance between the need to minimize the range of current-weighting resistance values and the need to reduce the number of separate components which must be assembled into a complete unit. It also represents a practical number of switches which can be provided with equal current densities in an IC configuration. Also of importance is the fact that quad-switches can readily be assembled to provide either a straight binary conversion, or a binary-coded-decimal (BCD) conversion, simply by selecting the appropriate currentdividing attenuation networks between the separate quad units.

Further refinements in performance can be obtained by using a compensation circuit as shown in FIG. 5. In this arrangement, a quad-switch module 10, like that in FIG. 1, is provided with an additional transistor 100 diffused in the substrate 12 (see also FIG. 2). This additional transistor serves as a reference for controlling the base supply voltage for the four switching transistors.

The reference transistor 100 .is identical to the LSB transistor 20, having a single emitter and a base like that of the LSB transistor. The resistor module 52 is provided with an additioanl 80K current-metering resistor 102 for the reference transistor. The base of the reference transistor isconnected to the common base rail 104 for the switching transistors.

The current flowing into the reference transistor 100 is fixed at exactly 0.125 ma. by a regulated current source comprising a Zener diode 106, a current-setting reference resistor 108, and an operational amplifier 110 the output of which drives the base of the reference transistor (and thereby drives the bases of all of the switching transistors due to the common base rail 104). The reference resistor 108 is trimmed to the precise value to produce 0.125 ma. into the collector of the reference transistor. Since the LSB switching transistor is identical to the reference transistor (same size emitter and base), and both are connected to identical resistors 50, 102 of the resistor module 52, the current into the LSB transistor also must be 0.125 ma., and the current densities must be equal. And since the other switching transistors are, in effect, exact multiples (binarily weighted) of the LSB transistor, the currents through those other transistors similarly will be exact binary multiples of the LSB current.

In more detail, the reference current established by zener diode 106 and reference resistor 108 is summed with the collector current of the reference transistor 100. The operational amplifier 110 adjusts the voltage of the common base line 104 to make the two currents equal. With the reference current thus properly established, all other currents will be in the correct proportion to one another, since the base-to-emitter drops (V of all of the 1C transistors will be equal.

Any changes in the parameters of the switchingtransistor circuitry will be compensated for by the feedback action of the amplifier 110, which will tend to hold constant the current into the collector of reference transistor 100, and thereby (in open loop mode) hold constant the actual output currents of the switching transistors. Any change in a circuit parameter which tends to alter the collector currents of the switching transistors 14-20 will similarly tend to alter the reference transistor collector current from equality with the originally established reference current. Such a tendency of the reference transistor collector current to change will be sensed at the input to amplifier 110, and its output will alter the base voltage of the reference transistor (and the bases of all of the switching transistors) so as to maintain the original current levels.

It should be noted that the reference transistor collector current corresponds identically to the currents of the switching transistors which must be held constant. That is, the output of the D-A converter is, basically, the collector currents of the activated switching transistors. Thus, by holding constant the reference transistor collector current, and thereby holding constant the switching transistor collector currents, the output of resistance values of such thin film resistors is :1 part per-million per C, so that any temperature-induced change in resistance of the current-weighting resistors 44-50 will be accompanied by a corresponding change in the resistance of resistor 102. The resulting tendency of the current through reference transistor 'l00 to change will be sensed by the operational amplifier 110,.

and its output will change correspondingly to adjust the base voltages so as to hold the currents constant.

Advantageously, the resistor module 52 also contains the power supply reference resistor 108 and the feedback resistor 114 of an output operational amplifier 116. If there is a change in resistance of the reference resistor 108, there will be a corresponding change in the collector currents of all of the transistors of the quad-switch, but this will be compensated for by a change in the resistance of the feedback resistor 114, so as to maintain the final output substantially constant.

FIG. 6 shows another compensation arrangement for the quad-switch module 10. Here the common base line for the switching transistors is tied to a fixed voltage (e.g., -3 volts), and the output of the reference amplifier 110 controls the voltage of the common line 122 to which all of the resistors are connected. In general, this compensation arrangement works on much the same basic principles as that of FIG. 5, in that the output of the reference amplifier is automatically controlled by sensing the differential between a reference current through resistor 108 and the collector current of the reference transistor 100. The amplifier in effect adjusts the voltage differential applied to the transistor base-to-emitter circuits so as to hold the collector currents constant.

The arrangement of FIG. 6 includes biasing circuitry used in a typical IC quad-switch module 10. This circuitry includes clamping diodes 124-130 to keep the buffer transistors 24-30 from saturating when those PNP devices are diverting the currents from the associated switching transistors. The circuit also incorporates transistors 132, 134, 136 for setting the base voltage of the buffer transistors.

FIG. 7 is a wiring diagram showing details of a complete D-A converter using three quad-switch modules 10A, 10B, 10C and a resistor module 52 interconnected by conductive elements of a printed circuit board. FIG. 8 illustrates the physical arrangement of the several components. Input connections to the converter are made through contacts 140 on one edge of the circuit board. The parallel digital input signal is directed through corresponding leads to three 4-bit registers 142, 144, 146 where the digital bits are strobed into respective storage stages for application to the input diodes of the switch control circuitry in each quad-switch.

The resistor module 52 contains all of the currentweighting resistors, the 16:1 current-dividing networks 82, 84, the power supply reference resistor 108, and the output feedback resistor 114. The reference amplifier 110 controls the common resistor line 122, as in the FIG. 6 arrangement discussed above. Although all quad-switch modules contain reference transistors 100,

tion of furnishing interconnections between the different components, as shown in the diagram.

Although preferred embodiments of the invention have been described hereinabove in detail, it is desired to emphasize that such details have been disclosed for the purpose of illustrating the nature of the invention, and should not be considered as necessarily limiting of the invention which can be expressed in many modified forms to meet particular requirements.

I claim:

1. An integrated-circuit digital-to-analog converter comprising:

a plurality of transistors on a single monolithic substrate;

switch control means operable by a digital input for selectively activating any of said transistors to pass current therethrough;

circuit means for said transistors to set different levels of current therethrough according to a predetermined weighting pattern such that the sum of the currents produced by the activated transistors represents an analog output in accordance with the required digital-to-analog relationship;

each of said transistors being formed to present in said substrate a respective conductivearea proportional to the particular current level assigned thereto in accordance with said weighting pattern, to provide uniform current density in said transistors when activated; and 1 means for summing the currents flowing through the transistors activated by said switch means so as to provide said analog output.

i 2. A converter as in claim 1, wherein said transistors are provided with different numbers of emitters proportional to the magnitude of current to be passed, said emitters all having equal areas.

' 3. A converter as in claim 1, including a second substrate having a plurality of resistors formed thereon; and

means interconnecting each of said transistors with a respective one of said resistors to contorl the current through each transistor in accoraance with the ohmic resistance of the associated resistor.

4. A co nverter as in claim 1, wherein said substrate is formed with four switching transistors the emitters of which have areas in the ratios of 1:2:4:8.

emittes a n 6 5 11 when!!! sai s tia has said plurality of transistors diffused therein to serve as separate current sources corresponding to,

respective bits of a digital input signal;

each of said transistors comprising an emitter, a

base and a collector; L the areas of the respective emitters for said transistors differing in accordance with a binary weighting pattern. 6. Apparatus as in claim 5, wherein each of said transistors has a number of separate equal-size emitter sections, with the number of sections in each case being in accordance with said binary weighting pattern.

7. Apparatus as in claims, wherein the size of the base segment for each transistor is at least approximately proportional to the area of the corresponding emitter.

8. Apparatus as in claim 7, wherein said substrate is formed with a single collector segment common to all of said transistors. 

1. An integrated-circuit digital-to-analog converter comprising: a plurality of transistors on a single monolithic substrate; switch control means operable by a digital input for selectively activating any of said transistors to pass current therethrough; circuit means for said transistors to set different levels of current therethrough according to a predetermined weighting pattern such that the sum of the currents produced by the activated transistors represents an analog output in accordance with the required digital-to-analog relationship; each of said transistors being formed to present in said substrate a respective conductive area proportiOnal to the particular current level assigned thereto in accordance with said weighting pattern, to provide uniform current density in said transistors when activated; and means for summing the currents flowing through the transistors activated by said switch means so as to provide said analog output.
 2. A converter as in claim 1, wherein said transistors are provided with different numbers of emitters proportional to the magnitude of current to be passed, said emitters all having equal areas.
 3. A converter as in claim 1, including a second substrate having a plurality of resistors formed thereon; and means interconnecting each of said transistors with a respective one of said resistors to contorl the current through each transistor in accoraance with the ohmic resistance of the associated resistor.
 4. A converter as in claim 1, wherein said substrate is formed with four switching transistors the emitters of which have areas in the ratios of 1:2:4:8.
 5. A converter as in claim 1, including a reference transisotr formed in said substrate; and circuit means responsive to current flowing through said reference transistor for automatically controlling the supply voltage for the other transistors so as to tend to prevent changes in current level in said other transistors.
 6. Apparatus as in claim 1, wherein said substrate has said plurality of transistors diffused therein to serve as separate current sources corresponding to respective bits of a digital input signal; each of said transistors comprising an emitter, a base and a collector; the areas of the respective emitters for said transistors differing in accordance with a binary weighting pattern.
 7. Apparatus as in claim 6, wherein each of said transistors has a number of separate equal-size emitter sections, with the number of sections in each case being in accordance with said binary weighting pattern.
 8. Apparatus as in claim 6, wherein the size of the base segment for each transistor is at least approximately proportional to the area of the corresponding emitter.
 9. Apparatus as in claim 8 wherein said substrate is formed with a single collector segment common to all of said transistors.
 10. A digital-to-analog converter comprising a single substrate formed with a plurality of switching transistors activatable in a selected pattern corresponding to a digital input signal; a power supply for producing a supply voltage for said switching transistors; circuit means for fixing the currents developed from each transistor in accordance with a weighting pattern based on the digital-to-analog relationship required; said substrate further being formed with a reference transistor matching one of said switching transistors and energized by said power supply; and sensing means responsive to the current flow through said reference transistor for adjusting the voltage produced by said power supply so as to tend to maintain the current through said reference transistor constant and thereby tend to maintain the currents through said switching transistor constant.
 11. Apparatus as in claim 10, wherein the conductive areas of said switching transistors are binarily weighted to produce a uniform current density in said switching transistors.
 12. Apparatus as in claim 11, wherein the conductive area of said reference transistor is selected to provide a current density equal to said uniform current density.
 13. Apparatus as in claim 12, wherein the conductive area of said reference transistor is equal to the conductive area of the switching transistor corresponding to the least significant bit of the digital signal applied to the corresponding set of switching transistors.
 14. Apparatus as in claim 10, wherein said current-fixing circuit means comprises a set of resistors on a second substrate and having ohmic resistances which are binarily weighted; and means connecting each of said transistors to one of said resistors to determine the current through each transiStor.
 15. Apparatus as in claim 14, wherein said resistors are formed from thin matallic deposits on a non-conductive substrate.
 16. Apparatus as in claim 14, wherein said sensing means comprises a current-weighting resistor on said second substrate and connected to said reference transistor to carry the current passing therethrough.
 17. Apparatus as in claim 10, wherein said power supply means includes a voltage source and a reference resistor connected thereto to establish a reference current; an operational amplifier connected to the outputs of said switching transistors to produce an analog output signal; a feedback resistor for said operational amplifier; said feedback resistor and said reference resistor being formed on a single common substrate, whereby the characteristics thereof are similar and will vary uniformly with changes in ambient temperature.
 18. Apparatus as in claim 17, wherein all of said resistors are formed on a single common substrate.
 19. In a digital-to-analog converter, the combination comprising: a circuit board; first and second separate and distinct substrates mounted on said circuit board, said substrates each providing an IC device presenting a plurality of selectively-operable transistor current switches and an additional transistor to serve as a reference for compensation purposes; digital input means to receive a digital input signal and coupled to said substrates to selectively activate said current switches in accordance with the digital input; said digital input means including first means to selectively activate the current switches of said first substrate in accordance with a first set of adjacent bits of the digital input signal, said digital input means further including second means to selectively activate the current switches of said second substrate in accordance with a second set of adjacent bits of the digital input signal; current-setting means coupled to said current switches to develop therefrom, when activated, individual output currents corresponding in magnitude to the order of the respective activating bit of the digital input signal; a common power supply for controlling the transistors on both substrates; and circuit means responsive to the current flowing through the reference transistor of one of said substrates, said circuit means including means to adjust said common power supply to tend to maintain the current through the reference transistor constant, thereby to tend to maintain constant the current through said transistor switches on both substrates; and analog output means coupled to both of said substrates to develop an analog output current responsive to the pattern of activation of the individual current switches of said substrates.
 20. Apparatus as in claim 10, wherein said transistors comprise base, emitter and collector electordes; said sensing means including means responsive to the current flow through the collector of said reference transistor for adjusting the voltage produced by said power supply to maintain the collector current constant.
 21. Apparatus as claimed in claim 20, including a plurality of resistors each connected at one end to the emitter of a respective one of said transistors to fix the currents passing therethrough; said power supply voltage being connected between the bases of all of said transistors and the other ends of said transistors, to adjust the current flow through said transistors with changes in said power supply voltage. 